hit man vs gun for hire

hit man and gun for hire both are nouns.

Word	Noun	Adjective	Verb	Adverb
hit man	Yes	No	No	No
gun for hire	Yes	No	No	No

As nouns, hit man and gun for hire are synonyms defined as:

Other synonyms of hit man include gun, gunman, gunslinger, hired gun, hitman, shooter, torpedo, triggerman.

hit man (noun)	gun for hire (noun)
a professional killer who uses a gun	a professional killer who uses a gun